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The Great Chicxulub Debate (2004)

The Great GSL Chicxulub Debate closed 25 January 2004. In response to sustained demand from its readers, the original presentation has been laboriously transferred from the Society's former website and rebuilt here, piece by piece. During this process some formatting (italics, superscripts etc) may have been lost. We are working to correct this.

To refer to this debate, please cite as follows:
Keller, G., Smit, J. et al.; The Geological Society of London Great Online Chicxulub Debate 2004.

The non-smoking gun

Gerta Keller

Ever since the discovery of the Chicxulub subsurface crater in the early l990s many scientists, the news media, Hollywood film-makers and the public have become convinced that a large meteorite killed the dinosaurs and most other organisms at the end of the Cretaceous. The theory is very attractive. The largest, most fascinating creatures ever to have roamed the Earth were apparently wiped out in a single day in a ball of fire caused by a meteorite that left behind the crater of doom just offshore of the Yucatan peninsula, Mexico. No wonder the minds and hearts of the public were captured. But is it true? Is there enough evidence to support this theory? We believe not. On the contrary, there is increasing evidence that the Chicxulub impact predated the end-Cretaceous mass extinction by about 300,000 years and did not cause the demise either of marine or terrestrial organisms1-3.

The Chicxulub impact was not the only thing making life horrible at the end of the Cretaceous. The mass extinction coincides in time with yet another - probably larger - impact right at the Cretaceous-Tertiary (K/T) boundary, which is well documented by virtue of its global iridium distribution. Moreover, massive volcanism that would one day create India's Deccan traps began during the late Maastrichtian and continued into the early Tertiary 4.5, causing major climate changes and long-term biotic stress. This stress, we believe, culminated in the K/T mass extinction - which we think was the combined effect of both volcanism and impacts. In this piece we summarise the evidence that challenges a K/T age for the Chicxulub impact, and the very notion that a single impact could be the sole cause of the K/T mass extinction.

Chicxulub is not K/T age

The Chicxulub crater has been linked to the K/T boundary mass extinction primarily on the evidence of impact glass spherules (microtektites) from Haiti and NE Mexico, and specifically:

39Ar/40Ar ages between 64.98 and 65.2 ± 0.2 Ma 6-7

geochemical similarity with melt rock from Chicxulub 8-9

stratigraphic proximity to the K/T boundary in localities throughout Mexico, Guatemala, Belize and Haiti1,2,11

the assumption that the siliciclastic unit that separates the spherule layers from the overlying K/T boundary in NE Mexico represents impact-generated tsunami deposits12-14.

We challenge 3 and 4.

Proximity to K/T

The impact spherule deposits are variable in their stratigraphic positions relative to the K/T boundary, which is globally identified by a specific set of criteria that includes:

a boundary clay and thin red layer with an Ir anomaly

a negative d13C shift and

the mass extinction of tropical and subtropical planktic foraminifera15.

For example, in Haiti, Belize, Guatemala and central and southern Mexico, the spherule deposits occur above the K/T boundary in the (early Tertiary) Danian zone Pla (named for Parvularugoglobigerina eugubina). They consist of spherules mixed with reworked late Maastrichtian marl clasts and planktic foraminifera 2,11. We consider these spherule deposits to be reworked, as shown in the Coxquihui section of central Mexico16 where an Ir anomaly is present above the spherule layer, as in Haiti and Guatemala (Fig. 1).

Fig. 1. The K/T transition at Coxquihui, Central Mexico, where the K/T boundary is marked by a hiatus and a thin (2cm) spherule layer. A 60cm thick spherule deposit is present in the early Danian zone Pla with an Ir anomaly above. This early Danian Ir anomaly has also been observed in Haiti and Guatemala and may represent an early Danian impact event.

In NE Mexico the spherule deposits occur in late Maastrichtian marls well below the K/T boundary. Up to four spherule layers have been discovered, interbedded in over 10m of late Maastrichtian marls below the siliciclastic unit and to date have been correlated over more than 25km (El Penon to Loma Cerca, Fig. 2) 1,2. Multiple spherule layers have also been documented from several dozen outcrops over an area of over 60km2 in five Master's theses.17-20 Only minor local slumps have been observed, which together with undisturbed marl sedimentation rules out, to our minds, a large-scale tectonic disturbance generated by the Chicxulub impact.

We interpret the lowermost spherule layer, which consists almost entirely of glass, as the original impact fallout deposit with subsequent layers reworked. This spherule layer occurs about 10m below the siliciclastic unit, and near the base of planktic foraminiferal zone CF1, which spans the last 300,000 years of the Maastrichtian. The oldest spherule layer thus predates the K/T boundary by nearly 300,000 years. Moreover, since the glass spherules have been geochemically linked to Chicxulub melt rock, this means that the Chicxulub impact itself predates the K/T boundary2.

Figure 2. Multiple impact glass spherule layers are present in late Maastrichtian marls of zone CF1 in NE Mexico and can be correlated across the entire region as shown here for El Penon and Loma Cerca. Zone CF1 spans the last 300 ky of the Maastrichtian. Glass spherules are generally closely packed (a) and may be fused and compressed (b) indicating deposition occurred while still hot and ductile.

Tsunami!

In l992 the only glass spherule layer known in NE Mexico occurred at the base of a thick siliciclastic unit bearing the K/T boundary at its top. The glass spherule layer could only be causally linked to the K/T boundary impact by interpreting the intervening siliciclastic unit as an impact-generated tsunami deposit 12-14.

This hypothesis became very popular even though there was contrary evidence from the beginning - namely, bioturbation (churning caused by burrowing organisms), rate of sedimentation and the position of the K/T boundary and Ir anomaly 21,22. The tsunami hypothesis was challenged during the l994 LPI-sponsored field trip (led by the present authors) by trace fossil expert Toni Ekdale, who discovered bioturbation within the upper siliciclastic unit.

Although he was effectively booed for this observation, he later returned to Mexico to study many of the classic K/T localities and document several horizons of bioturbation23. In El Penon, J-shaped burrows infilled with spherules were found near the base of the sandstone of layer (unit 2) and at the top of the sandy limestone layer that is within the underlying spherule layer (unit 1) in many sections. This indicates that the siliciclastic unit was deposited over a long time, during which the ocean floor was repeatedly colonised by invertebrates. It also suggests that the underlying spherule deposit does not represent the original fallout and that its re-deposition was interrupted by 'normal' deposition - of a bioturbated limestone layer (Fig. 3).

Figure 3. El Penon, Mexico. Impact spherule layer at base of siliciclastic deposit is separated by a 15-20 cm thick sandy limestone. J-shaped burrows infilled with spherules are present in the sandy limestone and sandstone unit above. This indicates that both the spherule and sandstone units were deposited over an extended time period that excludes tsunami deposition.

Chicxulub Cores

A new core, Yaxcopoil-1 (Yax-1), drilled within the crater, also suggests that the Chicxulub impact took place before the end of the Cretaceous. This is indicated by a 50cm-thick laminated, dolomitic limestone containing late Maastrichtian (zone CF1) planktic foraminifera, which occurs between the top of the suevite breccia and the K/T boundary 2. Yax-1 is not alone. Limestones containing late Maastrichtian planktic forams have been found to overlie the impact breccia in cores T1, Y6 and C1 24 - a fact also supported by e-log correlations 25 (Fig. 4).

Figure 4. Correlation of Yaxcopoil-1 (Yax-1) with Pemex cores from Yucatan[25]. Note that sedimentation, including the impact breccia, is reduced in Yax-1, suggesting that deposition occurred on an uplifted flank of the crater. The presence of late Maastrichtian sediments above the impact breccia has been observed in cores Yax-1, T1, Y6 and C1, as well as from NE Mexico. Stratified lithologies, normal age sequence and several biohorizons can be correlated across the crater (except for Y6 and C1) and onto the Yucatan platform, which indicates normal deposition.

Conclusion

The existence of an impact crater neither proves nor explains the demise of the dinosaurs, or the mass extinction of any other group. The palaeontological database amassed during the last 20 years lends little support to a scenario of sudden mass extinction at the K/T boundary 26, except for tropical-subtropical planktic foraminifera 27,28. Moreover, a sudden mass extinction cannot explain its selective nature, (e.g. foraminifera, rudists), its varying effects at different latitudes nor the progressive extinction trend that precedes the K/T boundary 26-29.

Can multiple impacts explain this trend? Although Chicxulub may now be considered a late Maastrichtian zone CF1 impact, coinciding with a time of major global greenhouse warming and volcanism, no major extinctions are associated with it. Was the K/T impact much larger, or did it coincide with major volcanism that together led to the final, lethal environmental disturbance? We believe the latter is the case.

A study of the way in which planktic foraminifera responded to late Maastrichtian volcanism on Ninetyeast Ridge (Indian Ocean), has revealed biotic effects virtually identical to the K/T mass extinction 30. To us, this is further indication that the end-Cretaceous mass extinction was the result not of a single hammer-blow. It was, instead, a progressive multi-event catastrophe - a concerted assault on the whole edifice of life by a combination of massive volcanism, multiple impacts and their associated climatic and environmental changes.

Stinnesbeck W. et al. 2002. The Cretaceous-Tertiary (K/T) boundary transition at Coxquihui, State of Veracruz, Mexico: evidence for an early Danian impact event? Journal of South American Research 15: 497-509.

Lopez Ramos E. l975. Geological summary of the Yucatan Peninsula, In The ocean basins and margins edited by Nairn A.E.M. and Stehli F.G. Vol. 3, The Gulf of Mexico and the Caribbean. New York, Plenum Press, 257-282.